Differential staining of human chromosomes can be obtained when the pH of Giemsa stain is changed to 9.0 from the usual 6.8. Such staining permits identification of all homolog pairs and distinct regions within chromosome arms. In most instances, the pattern is quite similar to that obtained with quinacrine mustard fluorescence staining. Certain regions, such as the paracentric constrictions in chromosomes Al and C9, and the distal end of the long arm of the Y chromosome stain differently with the Giemsa 9 technique. The technique is considerably simpler than the quinacrine mustard fluorescence technique and identification of homologs is also easier than in cells stained by the latter.